Can You Ever Suspend Wiring from Trees?

A few revisions to the 1999 National Electrical Code which are of interest and worthy of comment:

In Article 225, Outside Branch Circuits and Feeders, we find: "225-26. Vegetation. Vegetation such as trees shall not be used for support of overhead conductor spans. Exception: For temporary wiring in accordance with Article 305."

However, nowhere in Article 305, Temporary Wiring, is there any mention of whether temporary wiring can or cannot be supported from trees, and Section 305-2(a) states: "Except as specifically modified in this article, all other requirements of this Code for permanent wiring shall apply to temporary wiring installations." Since this subject is not specifically mentioned in Article 305, this becomes one of those circular references that gets you nowhere.

In the first paragraph of Section 305-4(c) we find: "Conductors shall be permitted within cable assemblies, or within multiconductor cord or cable identified in Table 400-4 for hard usage or extra-hard usage." In the second paragraph appears: "Branch circuits installed for the purposes specified in Sections 305-3(b) or (c) shall be permitted to be run as single insulated conductors." This is another example of the stampede in the code cycle to remove exceptions and make them into positive statements. That may be proper where the statements do not conflict, but they do here. In the 1993 Code there was an exception permitting single conductors for "Emergencies and Tests." The intent for the 1999 Code was to add to this the permission to use single conductors for the "90 Days" category of temporary wiring, reverting back to the pre- 1996 NEC for pumpkin lots, Christmas tree lots, etc. A worthy objective, but poorly presented when there are conflicting statements within the same section.

More confusion reigns in Section 310-15(b)(6) [Old Note 3] by the rewording which attempts to limit the application of Table 310-15(b)(6) to feeders which supply the loads in a dwelling unit that are traditionally subject to the diversity known, through years of experience, to exist. These smaller conductor sizes are practical and safe only when applied to a feeder that supplies the load of the dwelling unit as calculated by Part B of Article 220.

As applied to dwelling unit feeders, the permitted smaller conductor sizes have seesawed over the years. From the 1978 to 1987 Codes they applied to "three-wire, single-phase feeder that carries the total current supplied by that service." Then in the 1990 NEC the phrase "total current supplied by that service" was deleted. In the 1993 NEC the phrase was restored. In the 1996 NEC the phrase was changed to the "main power feeder to a dwelling unit." Finally, in the 1999 NEC we find: "(6) 120/240-Volt, 3-Wire, Single-Phase Dwelling Services and Feeders. For dwelling units, conductors, as listed in Table 310-15(b)(6), shall be permitted as 120/240-volt, 3-wire, single-phase service-entrance conductors, service lateral conductors, and feeder conductors that serve as the main power feeder to a dwelling unit and are installed in raceway or cable, with or without an equipment grounding conductor. For application of this section, the main power feeder shall be the feeder(s) between the main disconnect and the lighting and appliance branch-circuit panelboard(s), and the feeder conductors to a dwelling unit shall not be required to be larger than their service-entrance conductors."

The difficulty with this rewording is that, while the "main power feeder" is defined, it uses plurals for "feeder(s)" and "panelboards(s)." Only a single feeder and panelboard can see the diversity necessary to make these smaller conductor sizes work. I think we all know what is meant-perhaps some day it will say what it means.

In Ampacity Table 310-19 in the 1996 NEC there was a column for Bare and Covered conductors. If you missed it in the 1999 NEC, find new Table 310-21, taken from Table B310-4 in the appendix of the 1996 NEC. The ampacities are much higher than those shown in Table 310-19 in the 1996 NEC because they are calculated using the Neher-McGrath method and are to be used only under engineering supervision.

In the 1996 NEC, Section 346-12, there was an exception permitting vertical risers of rigid metal conduit to have 20 feet between supports, but limited to use with "industrial machinery." In the 1999 NEC this has been opened up by changing "industrial machinery" to "stationary equipment or fixtures." There are many locations in office buildings, schools, and hospitals, among others, where this is a practical method of support whether serving industrial machinery or not.

SCHWAN is an electrical code consultant in Hayward, Calif. He can be reached at bev-schwan@aol.com.